Regulations set forth malfunction criteria for engine systems. Among those is the requirement that the on-board diagnostics (OBD) system monitor the functions and conditions of the emissions control system, including engine catalysts (also known as catalytic converters). Engine catalysts reduce exhaust emissions by chemically converting exhaust into less harmful byproducts. Engine catalysts store oxygen, usually while the system is running fuel lean (oxygen rich). The stored oxygen is later released to compensate for a lack of oxygen while the system is running fuel rich (oxygen lean).
Oxygen storage capacity (OSC) refers to an engine catalyst's ability to store oxygen. Oxygen sensors located before and after an engine catalyst monitor the oxygen concentration of exhaust entering and exiting the catalyst. The oxygen sensors supply feedback signals relative to the oxygen concentration of the exhaust to the engine control module (ECM). The collected data is used to calculate the OSC, which is then used to monitor the performance of an engine catalyst.